The present invention is related to gas turbine engines, and in particular to variable stator vanes and variable stator vane actuation mechanisms.
Gas turbine engines operate by combusting fuel in compressed air to create heated gases with increased pressure and density. The heated gases are used to rotate turbines within the engine that are used to produce thrust or generate electricity. For example, in a propulsion engine, the heated gases are ultimately forced through an exhaust nozzle at a velocity higher than which inlet air is received into the engine to produce thrust for driving an aircraft. The heated gases are also used to rotate turbines within the engine that are used to drive a compressor that generates compressed air necessary to sustain the combustion process.
The compressor and turbine sections of a gas turbine engine typically comprise a series of rotor blade and stator vane stages, with the rotating blades pushing air past the stationary vanes. In general, stators redirect the trajectory of the air coming off the rotors for flow into the next stage. In the compressor, stators convert kinetic energy of moving air into pressure, while, in the turbine, stators accelerate pressurized air to extract kinetic energy. Gas turbine efficiency is, therefore, closely linked to the ability of a gas turbine engine to efficiently direct airflow within the compressor and turbine sections of the engine. Airflow through the compressor and turbine sections differs at various operating conditions of the engine, with more airflow being required at higher output levels. Variable stator vanes have been used to advantageously control the incidence of airflow onto rotor blades of subsequent compressor and turbine stages under different operating conditions.
Variable stator vanes are typically radially arranged within at least one shroud, which permits the vanes to rotate about trunnion posts at their innermost and outermost ends to vary the pitch of the vane. Typically, the outermost trunnion posts include crank arms that are connected to a synchronization ring, which is rotated by an actuator to rotate the vanes in unison. The outermost trunnions extend through the shroud, which could be an engine case, such that the synchronization ring is positioned outside of the shroud, while the vane airfoils are within the shroud, in the stream of the gases flowing through the engine. The engine case comprises a rigid structural component necessary for containing the high operational pressures of the engine, while the synchronization ring only requires enough stiffness to transmit torque to the crank arms. As such, the synchronization ring has a tendency to deform, due to chording and localized force reactions, when acted upon by the actuator as the synchronization ring is suspended over the engine case by the crank arms. These deformations cause an undesirable error in the positions of the vanes controlled by that ring. Typically, runners are positioned between the synchronization ring and the engine case to locally and radially support the synchronization ring. The runners link the synchronization ring to the engine case such that the engine case lends its stiffness to the synchronization ring, thus retaining the centricity of the synchronization ring. By reducing the deflections of the ring and maintaining centricity, the error in the vane positions is greatly reduced, thereby improving engine performance and operability.
Because the synchronization ring is disposed outside of the engine case and the flow of the gases, the synchronization ring is less affected by the temperature of the gasses. This results in the engine case expanding and contracting in response to a change in the gas temperature more quickly than the synchronization ring does. This uneven expansion and contraction between the synchronization ring and the engine case can put stress on the runners that can eventually lead to their failure.